This invention relates to a process for smelting of silicon using a plasma as a heat source and is particularly directed to the preparation of silicon at purities adequate for metallurgical use and for use in solar cells.
At present, silicon is typically produced in a submerged electric arc furnace via the carbothermic reduction of silicon dioxide (SiO.sub.2) with a solid carbonaceous reducing agent. The silicon dioxide may be in the form of quartz, fused or fume silica, or the like. The carbonaceous material may be in the form of coke, coal, wood chips, and other forms of carbon containing materials. The overall reduction reaction being EQU SiO.sub.2 +2C=Si+2CO.
It is generally recognized that the above reaction in reality involves multiple reactions, the most significant being outlined below: EQU SiO.sub.2 +3C=SiC+2CO (1), EQU SiO.sub.2 +C=SiO+CO (2), EQU SiO+2C=SiC+CO (3), EQU 2SiO.sub.2 +SiC=3SiO+CO (4),
and EQU SiO+SiC=2Si+CO (5),
Silicon monoxide (SiO) is a gaseous species at the temperature of reaction and can be lost as a vapor if not completely reacted. Muller et al., Scand. J. Metall., 1 (1972), pp. 145-155, describe and define the theoretical equilibrium conditions for the Si--O--C chemical system of the carbothermic reduction of silicon dioxide to form silicon. A critical teaching of Muller et al. is the limitation that under equilibrium conditions the partial pressure of silicon monoxide must be equal to or greater than 0.67 atmospheres at atmospheric pressure and at a temperature of 1819.degree. C. for reaction (5), above, to occur to form silicon. Johannson and Eriksson, J. Electrochem. Soc.:SOLID STATE SCIENCE AND TECHNOLOGY, 131:2 (1984), pp. 365-370, further expand upon the description and definition of the Si--O--C system. The teachings of Johannson and Eriksson define the influence of pressure upon the reaction. It is shown, theoretically, that 5 atmospheres is an optimum pressure for maximizing raw material efficiency to essentially a 100% silicon yield.
The use of a submerged electric arc furnace for the production of silicon has been used on a commercial basis for many years. It is generally recognized that there are several inherent disadvantages in use of such a system. In the present use of the submerged electric arc furnance, the silicon dioxide and carbonaceous reaction solids are charged to the top of the furnace. As the reaction progresses, a cavity forms at the bottom of the furnace at the lower end of the submerged electrode. Molten silicon collects at the bottom of the cavity. At the top of the cavity is a crust of reactants, intermediates, and product silicon. Above this crust are varying forms of solid reactants and intermediates.
Poor heat and mass transfer in a submerged electric arc furnace appear to cause poor utilization of the electrical energy applied and lowered raw material utilization. Present commercial units consume approximately 3 times the theoretical amount of energy required for these above reactions. This high level of energy consumption reflects the loss of energy introduced with the carbonaceous reductants as carbon monoxide lost in the by-product off-gases. Several factors contribute to the poor heat and mass transfer. The solid-solid and solid-gas mass transfer interactions between reactants and intermediates in the furnace limit effective heat and mass transfer in a conventional arc furnace. A further disadvantage is the loss of material in the form of volatile SiO with the gaseous by-products of the reaction. It is estimated that in present submerged arc furnaces, as much as 10 to 20 weight percent of the ultimate silicon yield is lost as SiO. Silicon monoxide reoxidizes to form SiO.sub.2. As a consequence the SiO poses problems not only of material loss but plugging problems throughout the process. Further, SiO.sub.2 that escapes from the system poses an environmental problem as an airborne particulate that must be collected and discarded, with considerable difficulty.
The present submerged electric arc furnace route to silicon is also hampered by mechanical problems. The flow of solids moving downward, counter-current to the flow of gases moving upward inhibits the flow of solids to the reaction cavity. Additionally solids are held up by bridging which is caused by the formation of the crust above the reaction cavity and the proximity of solids to the vertical electrodes. Bridging is also caused by the formation of sticky intermediates in the cooler upper portion of the furnace. This hold-up of solids necessitates the inclusion of openings in the furnace top and frequent opening of the reactor and rodding or "stoking" of the solids to facilitate a downward movement.
The carbon electrodes of the arc furnace are consumed and contribute both to the impurities in the final product silicon and the final cost of manufacture. The carbon electrodes are the major source of impurities in preparation of silicon in a conventional arc furnace. Further, it is estimated that as much as 10% of the cost of silicon manufacturing is attributable to replacement of and problems associated with the electrodes.
The use of a plasma in place of an electric arc furnace has several advantages. According to the reaction scheme, described supra, reaction (1) EQU SiO.sub.2 +3C=SiC+2CO
is endothermic and consumes as much as 50% of the energy for the overall reduction reaction. Feeding of SiO.sub.2 and carbon-containing material directly into the high-energy plasma maximizes heat and mass transfer to facilitate this reaction to form SiC. The efficient formation of SiC would further facilitate the subsequent reaction chain to form silicon, represented by the reactions (4) and (5), supra, EQU 2SiO.sub.2 +SiC=3SiO+CO,
and EQU SiC+SiO=2Si+CO.
The simultaneous melting of SiO.sub.2 and formation of SiC would improve mass transfer. Configurational changes in the reactor could also eliminate the bridging of solids and the need to periodically open the furnace for "stoking." As a consequence, the furnace could be closed and operated under pressure. Closing of the furnace would facilitate recovery and reclamation of the energy content of the by-product gases, presently lost as noted, supra. The elimination of the carbon electrodes used in an arc furnace would result in subsequent increased purity of the final silicon product.
The use of a plasma to treat metal oxides is taught by Foex in U.S. Pat. No. 3,257,196, issued June 21, 1966. The method taught by Foex is the compressing of the material to be treated in a vessel which is capable of being rotated on its center axis. An axial cavity is provided into which the plasma can penetrate. The plasma may be used as a vehicle to carry reactants to the zone of solid reactants. The teachings of Foex are built around the need for a rotatable reactor which is obviously in a complicated batch configuration as compared to the continuous scheme for the instant invention. Additionally, the teachings of Foex are directed to eliminating the need for maintaining a powdered metal oxide feed in the plasma jet by compressing said powder into said rotating reactor and utilizing the centrifugal force to retain the powder in the reactor. The reaction zone in the Foex teaching would be at the surface of a dense, compacted solid rather than through a porous bed of solids as disclosed in the instant invention. The instant invention teaches the continuous feed of powdered reactants into the plasma zone. These differences would have a significant impact upon improved efficiency of mass and heat transfer for the instant invention.
Coldwell and Roques, J. Electrochemical Soc., 124(11) (1977), pp. 1686-1689, describe the reaction of a rod of pressed silicon dioxide and carbon powder in a plasma. Coldwell and Roques also describe the use of a radio-frequency induced plasma. As will be discussed, infra, the high gas flows associated with an induced plasma pose a severe limitation on the reduction reaction to form silicon. Further, Coldwell and Roques describe the difficulties caused by the high gas flows needed for the induced plasma. The product silicon was a vapor which was recovered by quenching. Silicon was never more than 33% of the quenched product. This low silicon recovery was thought by Coldwell and Roques to be the best attainable because of the high reactivity of the species that were formed in the plasma at the given conditions. The method of Coldwell and Roques is a batch procedure as compared to the continuous process of the instant invention. Additionally, Coldwell and Roques were obviously working in a much higher temperature regime than the instant invention, given the fact that silicon left the reaction zone as a vapor. This higher temperature regime completely changes the chemical and thermal equilibria of the system and makes comparison to the instant invention meaningless.
Stramke et al. in German OLS No. 2,924,584, published on Jan. 15, 1981, describe the passing of silica or silicon through a plasma flame in a reducing atmosphere. The teaching of Stramke et al., is not directed to the carbothermic reduction of silicon dioxide, as is the instant invention, but rather to the reduction of impurities in silica or silicon so that these reduced impurities can be volatilized and removed from the silicon material. Reducing gases cited were hydrogen (H.sub.2), methane, ethane, and ethylene, and other saturated and unsaturated lower hydrocarbons.
Dahlberg et al., in U.S. Pat. No. 4,377,564, issued on Mar. 22, 1983, describe preparation of silicon in a plasma using silicon dioxide and a reducing agent. Silicon is produced in a plasma as a vapor and is recovered from the vapor reaction mixture by deposition on a substrate or condensation. No mention is made of yields. However, it would appear that this teaching would have the same shortcomings as those of the method of Coldwell and Rogues, supra. Reducing agents cited were carbon, hydrogen, hydrocarbons, nitrogen, carbon monoxide (CO), halogens, and water vapor.
Santen and Edstrom in U.S. Pat. No. 4,439,410, issued Mar. 27, 1984, disclose a process for preparing silicon in which silica and an optional reducing agent are injected into a gas plasma. The heated feed and energy-rich plasma gas are introduced into a reaction chamber packed with a solid reducing agent. Silica is caused to melt and is reduced to silicon. Reaction gases comprise a mixture of H.sub.2 and CO and can be recirculated and used as a carrier gas for the plasma. Santen and Edstrom disclose that the plasma can be generated by electrical arc or inductive means. Reducing agents cited were hydrocarbon (natural gas), coal dust, charcoal dust, carbon black, petroleum coke, and others.
In studying the Santen and Edstrom patent, several inconsistencies are noted. First, the description of the invention discloses that the plasma burner used is an inductive plasma burner. Secondly, the description of the invention is silent on the generation of a plasma by electric arc means which is, however, claimed. Santen and Edstrom claim that the plasma is also generated by allowing a plasma gas to pass an electric arc. Santen and Edstrom are silent as to whether or not the plasma is generated in a transferred arc or a non-transferred arc mode which indicates that they did not appreciate the significant differences which lead to the benefits derived from the instant invention. This distinction is very significant. The transferred arc mode uses a minimum of gas, while the non-transferred arc mode utilizes a gas volume that is approximately 5 to 10 times greater to transfer a like amount of energy. As an example of the difference in gas volume required, for a plasma generated with 1000 kilowatts (kW) of energy a transferred arc configuration would require 10 to 25 standard cubic feet per minute (scfm) of gas compared to 100 to 150 scfm or more required for a non-transferred arc configuration. In the transferred arc mode, two electrodes are spaced a distance apart, such as the top and the bottom of the reactor. The plasma gases can flow either from the cathode to the anode or vice versa. The volume of gas utilized in the transferred arc mode is that volume necessary to form the plasma itself. In the non-transferred arc mode, two electrodes are in the generator itself. The arc is struck in the generator, the plasma is formed, and the plasma is in effect blown into the reaction zone by a larger volume of gas. In a non-transferred arc configuration, it is estimated that 10% of the feed gas is converted to a plasma, while 90% of the feed gas is used to move the plasma into the reaction zone. A radio-frequency induced plasma utilizes the same relative volume of gas per level of input energy as does the non-transferred arc plasma. In regard to the use of an inductive plasma burner, other references in the art (as an example, National Institute for Metallurgy Report No. 1895, "A Review of Plasma Technology with Particular Reference to Ferro-Alloy Production," Apr. 14, 1977, pg. 3) note that the scale-up of radio-frequency induced plasmas is difficult and expensive and remains essentially a laboratory tool. The dilution by an extraneous gas can severely reduce the partial pressure of the silicon monoxide intermediate and inhibit the formation of silicon, as noted in the reference of Muller et al., supra. This phenomenon will be discussed and shown in the examples, infra.
As a further inconsistency, Santen and Edstrom teach the use of recycled H.sub.2 and CO as the plasma gas. It was found in the development of the instant invention that addition of CO to the reaction zone severely inhibited the formation of silicon. The significance of this finding will be discussed in the examples, infra.
Several significant findings were discovered during the development of the instant invention. It was found that use of a plasma in a non-transferred arc configuration in which the plasma gases and a continuous feed of silicon dioxide and solid carbonaceous material was passed through the reaction bed of solids resulted in no silicon formation. The high flow of plasma gases would have a significant impact upon dilution of the reaction gases. This finding is consistent with the teachings, supra, which indicate that silicon will not form until a critical partial pressure of silicon monoxide is exceeded. To further illustrate this phenomenon, a modification to the plasma-reactor configuration in which the plasma gases did not penetrate the reaction bed and did not subsequently dilute the reaction gases resulted in the formation of silicon. This modification, discussed in the example, infra, would approximate the gas flow in the reaction zone for a transferred arc plasma configuration.
A further finding was the demonstration that addition of carbon monoxide to the reaction zone of a reactor which was producing silicon stopped the formation of silicon. This finding is illustrated in the example, infra.